1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier stage. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier stage amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage demodulates the filtered signal to recover the raw data in accordance with the particular wireless communication standard.
The most widespread communication standard in the area of wireless personal area networks (PANs) is currently Bluetooth. This communication standard employs Gaussian minimum shift keying (GMSK), which is a constant-envelope binary frequency shift keying (FSK) modulation scheme allowing raw transmission at a maximum rate of 1 Megabits per second (Mbps). While standard Bluetooth is sufficient for voice services, future high-fidelity audio and data services demand higher data throughput rates. Higher data rates can be achieved in the specification of the Bluetooth Enhanced Data Rates (Bluetooth EDR) standard by selectively applying a 4-level or 8-level phase shift keying (PSK) modulation scheme. With these variable-envelope communication scheme options, the maximum bit rate is increased 4-fold or 8-fold, respectively, compared to standard Bluetooth, while the chosen pulse shaping, a square-root raised cosine filter with a roll-off factor of 0.4, ensures that the RF carrier bandwidth is the same as that of standard Bluetooth, allowing for the reuse of the RF frequency channels.
Square Root Raised Cosine (RRC) filters are popular in wireless transceivers since, as is well-known, provided the transmitter implements pulse shaping with an identical filter, the receiver can sample the transmitted signal without inter-symbol interference, and hence improve the resistance towards noise and interferers of the system. However, typically, it is not possible to implement an RRC filter directly in the receiver signal path. This is due to the fact that some amount of analog filtering in the receiver is needed prior to analog-to-digital conversion, and the fact that RRC filters require implementation of lengthy hardware in-efficient FIR filters with many multipliers. Instead, linear RX path equalizers are most often implemented in the receiver to “un-distort” the receiver signal path in such a way that the total combined filtering of the receiver over the signal bandwidth closely resembles that of an RRC filter, while still providing a hardware efficient solution.
A critical parameter of current PSK demodulator design is the computational complexity, specifically the number of multiplications, required to implement the equalization functions of the RX path equalizer. In general, the optimal sampling point of a signal is not known a-priori. Thus, in order to ensure that the demodulator has access to the optimal sampling point, the RX path equalizer output needs to be generated at finely spaced points in time. Typical over-sampling ratios required for high performance are 12 or 16. This, in turn, determines the computational complexity of the RX path equalizers. For example, an RX path equalizer may require 8 multiplications per output sample. Thus, operating the equalizer at 12 MHz requires a total of 192 million Mults/second. This arithmetic complexity is substantial and contributes to the fact that the PSK demodulator requires relatively large die area (hardware) and relatively large power consumption.
Therefore, a need exists for a hardware-efficient and power-efficient PSK demodulator design for use in receivers.